Such a circuit arrangement is disclosed in DE 103 45 610 A1 and is illustrated in FIG. 1 for the purpose of easing comprehension. Said figure shows a circuit arrangement with two field effect transistors T1, T2 that are arranged in the manner of a half bridge inverter. The two field effect transistors receive their control signal from a microcontroller MC. An intermediate circuit capacitor C1 with a comparatively large capacitance is arranged in parallel with the DC input voltage of the half bridge inverter T1, T2. The intermediate circuit capacitor C1 serves as DC voltage source and provides the so-called intermediate circuit voltage UZw for the half bridge inverter. The intermediate circuit voltage UZw is usually approximately 400 V and is generated on the AC voltage by means of a system voltage rectifier (not illustrated) and of a boost converter (not illustrated). The intermediate circuit capacitor C1 is arranged in parallel with the voltage output of the boost converter. Connected to the output M of the half bridge inverter is a load circuit that is designed as a series resonant circuit and consists essentially of the lamp inductor L1 and the ignition capacitor C2. Connected in parallel with the ignition capacitor C2 are the discharge paths of the fluorescent lamp LP and the capacitor C3, which is charged up to half the supply voltage of the half bridge inverter during operation of the lamp in the steady state of the half bridge inverter. The lamp electrodes E1, E2 of the fluorescent lamp LP are designed as electrode coils each having two electrical connections. Connected in parallel with the electrode coils E1, E2 in each case is a secondary winding SI1, SI2 of a transformer that serves the inductive heating of electrode coils E1, E2. The primary winding P1 of this transformer is connected in series with the switching path of a further field effect transistor T3 to whose control electrode the microcontroller MC likewise applies control signals, and a measurement resistor R1, during dropping across the measurement resistor R1 is a voltage Res that is correlated with the reciprocal of the electrical resistance of a coil E1, E2 of the discharge lamp LP. The series connection of the components P1, T3 and R1 is connected to the output M of the half bridge inverter. A first connection of the primary winding P1 is connected to the output or the center tap M of the half bridge inverter and to the lamp inductor L1, while the second connection of the primary winding P1 is connected to the field effect transistor T3 and, in the DC forward direction via a diode D1 to the connection (+) at a high potential, of the intermediate circuit capacitor C1. A first connection of the measurement resistor R1 is connected to frame potential (−), while the second connection of the measurement resistor is connected to the field effect transistor T3 and, via a low pass filter R2, C4, to the voltage input A of the microcontroller MC.
By means of the coupling capacitor C3 charged up to half the supply voltage of the half bridge inverter, and of the alternately switching transistors T1, T2 of the half bridge inverter, a high frequency AC voltage is applied to the load circuit L1, C2, LP in a known way, its frequency being determined by the switching cycle of the transistors T1, T2, and is in the range of approximately 50 kHz to approximately 150 kHz. Before the ignition of the gas discharge in the fluorescent lamp LB, a heating current is applied to the lamp electrodes E1, E2 thereof by means of the transformer P1, SI1, SI2 in an inductive fashion. For this purpose, the transistor T3 is switched on and off by the microcontroller MC in a fashion synchronous with the transistor T1. In the course of the switched-on duration of the transistors T1, T3, a current therefore flows through the primary winding P1 and the measurement resistor R1. In the course of the switched off duration of the transistors T1, T3, the flow of current through the measurement resistor R1 is interrupted. The energy stored in the magnetic field of the primary winding P1 is fed to the intermediate circuit capacitor C1 via the diode D1 in the course of the switched-off duration of the transistors T1, T3 and the switched-on duration of the transistor T2. Owing to the alternately switching transistors T1, T2 and to the transistor T3 switching synchronously with the transistor T1, a high frequency current flows through the primary winding P1 and induces corresponding heating currents for the electrode coils E1, E2 in the secondary windings SI1, SI2. The voltage drop across the measurement resistor R1 over a time interval of a plurality of switching cycles of the transistor T3 is averaged with the aid of the low pass filter R2, C4 and fed to the voltage input A of the microcontroller MC. The input voltage at the connection A of the microcontroller MC is converted by means of an analog-to-digital converter into a digital signal and evaluated in the microcontroller MC.
The microcontroller MC detects the voltage drop across the capacitor C4 for the first time after approximately 30 ms after the beginning of the heating phase, and for the second time approximately 600 ms after the beginning of the heating phase. If the absolute value of the difference between the two voltage values exceeds a prescribed threshold value, the voltage value at the end of the heating phase is compared with a reference value stored in the microcontroller MC and used for the lamp-type recognition. As already mentioned, in this case the voltage value is correlated with the reciprocal of the coil resistance. If the absolute value of the difference between the two voltage values is less than the threshold value, the lamp continues to be operated with the current data set, that is to say no lamp-type recognition is carried out. The latter is the case in accordance with the publication named when the electrode coils E1, E2 have not yet been entirely cooled at the beginning of the heating phase owing to the last lamp operation, or when the circuit arrangement is operated with an ohmic dummy resistance instead of the electrode coils E1 and E2 of the fluorescent lamp LP.
In accordance with a further prior art, which is used by the applicant in circuit arrangements already marketed, a further evaluation of the measured coil resistances such as is illustrated in conjunction with FIG. 2 is undertaken on the basis of the prior art in accordance with DE 103 45 610 A1. The aim of this procedure is to detect one or more coil short circuits owing to instances of incorrect wiring of the luminaires in the case of electronic circuit arrangements. The aim of this approach is to avoid instants of coil darkening or the occurrence of damage to the circuit arrangement during operation.
The known method starts in step 100. Subsequently, a check is made in step 110 as to whether the intermediate circuit voltage UZw has reached its desired value UZwsoll. If this is not the case, the intermediate circuit voltage UZw is increased in step 120. If it is determined in step 110 that the intermediate circuit voltage UZw has reached its desired value UZwsoll, a first value Res1new of the voltage drop at the measurement resistor R1 that is correlated with the coil resistance of a coil of the fluorescent lamp LP is determined in step 130 at a first instant t1, and a second value Res2new of this voltage drop is determined at a second instant t2. In step 140, the difference (Res1new−Res2new) is compared with a first threshold value S1. If the difference is greater than the threshold value, an algorithm for lamp-type recognition is carried out. Said algorithm comprises the steps 150 to 230. In this process, the absolute value
                      Re        ⁢                                  ⁢        s        ⁢                                  ⁢        2        ⁢                                  ⁢        new                    Re        ⁢                                  ⁢        s        ⁢                                  ⁢        2        ⁢                                  ⁢        old              -    1    is firstly compared in step 150 with a threshold value X1, Res2new constituting the currently measured value of the voltage drop across the measurement resistor R1, and Res2old the value of the preceding measurement. If the absolute value
                      Re        ⁢                                  ⁢        s        ⁢                                  ⁢        2        ⁢                                  ⁢        new                    Re        ⁢                                  ⁢        s        ⁢                                  ⁢        2        ⁢                                  ⁢        old              -    1    is less than the threshold X1, the lamp is operated in step 160 with the current set of operating parameters. The new value Res2new differs only very slightly from the old value Res2old, and so there is no doubt that the same lamp is connected to the circuit arrangement. Consequently, said lamp can be operated without change in step 160 with the aid of the current data set. If, by contrast, the value
                      Re        ⁢                                  ⁢        s        ⁢                                  ⁢        2        ⁢                                  ⁢        new                    Re        ⁢                                  ⁢        s        ⁢                                  ⁢        2        ⁢                                  ⁢        old              -    1    is greater than the threshold X1, it is determined in step 170 whether the value
                      Re        ⁢                                  ⁢        s        ⁢                                  ⁢        2        ⁢                                  ⁢        new                    Re        ⁢                                  ⁢        s        ⁢                                  ⁢        2        ⁢                                  ⁢        old              -    1    lies between the threshold X1 and a threshold X2, X2 being greater than X1. If this is affirmed, it is assumed that the same lamp is continued to be referred to, but has only aged a little. Consequently, the new value Res2new is overwritten on the old value Res2old in step 180. Thereafter, the lamp continues to be operated with the aid of a current data set in step 190.
If, by contrast, it is determined in step 170 that the value
                      Re        ⁢                                  ⁢        s        ⁢                                  ⁢        2        ⁢                                  ⁢        new                    Re        ⁢                                  ⁢        s        ⁢                                  ⁢        2        ⁢                                  ⁢        old              -    1    does not lie between X1 and X2, the value of Res2new is looked up in a table in order to derive therefrom the lamp type to which this Res2new is assigned. If the corresponding lamp data set is recognized in step 200 in this case, the lamp is operated in step 210 with the aid of the detected lamp data set i. Res2new is overwritten on Res2old in step 220. If no lamp data set for Res2new is found in step 200, the lamp is operated with a default data set in step 230.
If it is determined in step 140 that the difference between Res1new and Res2new is below the threshold value S1, a check is made in step 240 as to whether the difference (Res1new−Res2new) lies below a second threshold value S2 that is less than the threshold value S1. If this is the case, a dummy coil is assumed in step 250, or a coil short circuit. If a dummy coil can be excluded (it being the case that a lamp is used), a coil short circuit is therefore present and the circuit arrangement is switched off. If it is determined in step 240 that the difference between Res1new and Res2new is greater than the threshold S2, the lamp continues to be operated in step 260 with the current data set.
It has now been determined that damage to the circuit arrangement occurs repeatedly in the case of the procedure outlined when the plurality of the luminaires are operated simultaneously in a single circuit arrangement.